The present application relates to a battery pack, a method of charging a secondary battery, and a battery charger. More particularly, the application relates to a battery pack, a method of charging a secondary battery, and a battery charger, which control a charge condition in accordance with the deterioration coefficient of a secondary battery.
A combination of constant-current charge and constant-voltage charge (constant-current constant-voltage method) is known as a method of charging a secondary battery. This charging method will be described with reference to FIG. 9, on which the abscissa represents charging time and the ordinate represents cell voltage and charging current. In FIG. 9, the region indicated by arrows a and b (hereinafter referred to as a “region a-b”) is the range of constant-current charge, and the region indicated by arrows c and d (hereinafter referred to as a “region c-d”) is the range of constant-voltage charge. The arrow I indicates charging current, and the arrow V indicates cell voltage. A power supply section for charging performs the operation of constant-current control in the region a-b, and performs the operation of constant-voltage control in the region c-d.
As shown in FIG. 9, during the region a-b, the constant-current charge is performed by a predetermined current value, and then the cell voltage rises. In this example, the charging current is maintained at, for example, 3300 mA. When the charging proceeds, so that the cell voltage reaches a predetermined voltage value, for example, 4100 mV, switching takes place from constant-current charge to constant-voltage charge. During the region c-d, the charging current is gradually lowered, and the cell voltage rises toward the output voltage (e.g. 4200 mV) of the power supply section. Thereafter, the charging is completed when the charging current becomes smaller than a predetermined value (a current value at end-of-charge).
The secondary battery gradually deteriorates with increasing the number of charge and discharge cycles, where each cycle consists of charge, and discharge and pause, as described above. The secondary battery deterioration has been determined by the count of charge and discharge cycles, the actual discharge amount, or charging time. For example, in the method using the count of charge and discharge cycles, the number of charge and discharge cycles is calculated by using charging amount or discharging amount, and a deterioration coefficient is obtained from the number of charge and discharge cycles. When the actual discharging amount is used, a deterioration coefficient is obtained from the actual discharging amount and a design capacity value by using a calculation equation: Deterioration coefficient=Design value capacity/Actual discharging amount.
Japanese Unexamined Patent Application Publication No. H11-174136 discloses the method including the steps of measuring the effective resistance in a battery pack and determining the deterioration thereof by the measured value. When the effective resistance is used, the deterioration can be determined in a short time.